In general, a thin film field effect transistor comprises an island or area, generally rectangular, of polycrystalline silicon (hereinafter referred to as "polysilicon") having a thin gate insulator layer, generally of silicon dioxide, extending over a portion thereof. A conductive gate layer, generally of doped polysilicon, is over the gate insulator layer. Conductive contacts, generally of a metal, extend over and contact the portions of the polysilicon layer on opposite sides of the gate layer to contact the source and drain of the transistor.
Referring now to FIGS. 1 and 2, there is shown a perspective view (FIG. 1) and cross-sectional view (FIG. 2) through line 2--2 of FIG. 1 of a prior art thin film field effect transistor 10. The thin film field effect transistor 10 comprises a substrate 12 of insulating material, such as silicon dioxide, having thereon an island (area) 14 of polysilicon. The silicon dioxide of the substrate may be a layer on a body of another material, such as a plate of glass or a body of silicon. Extending across a portion of the polysilicon island 14 is a thin gate insulator layer 16 which is typically of silicon dioxide. The gate insulator layer 16 extends across a top surface 18 of the polysilicon island 14 and a pair of opposed side edges 20 thereof. A conductive gate 22, typically of doped polysilicon, is over the gate insulator layer 16. The portions of the polysilicon island 14 at opposite sides of the gate 22 are doped to form a source region 23 and a drain region 25 of the transistor 10. Conductive contacts (electrodes) 24 and 26, generally of a metal, extend over and engage the source region 23 and the drain region 25, respectively, of the transistor 10. A conductive contact (electrode) 28 also extends over and contacts the gate 22.
Referring now to FIGS. 3-6, there are illustrated the steps of a typical method for making the thin film field effect transistor 10. As shown in FIG. 3, a layer 30 of polysilicon is first deposited on the substrate 12. A mask 32 of a material which is not attacked by an etchant for polysilicon, such as photoresist, is then formed over the area of the polysilicon layer 30 which is to form the island 14 using standard photolithographic techniques. As shown in FIG. 4, the exposed area of the polysilicon layer 30 is then removed using a suitable etching technique, such as plasma etching, to leave the polysilicon island 14. The mask 32 could be made of silicon dioxide and a chemical etch could be used. As shown in FIG. 5, the silicon dioxide gate layer 16 is then formed on the top surface 18 and edges 20 of the island 14 by heating the polysilicon island 14 in an oxygen atmosphere at a temperature at which the polysilicon is oxidized to form the silicon dioxide layer on the island 14. As shown in FIG. 6, a gate layer 22 of doped polysilicon is then deposited on the gate insulator layer 16.
A problem with the thin film field effect transistor 10 which results from the above-described method of making it, is that the gate insulator layer is not of uniform thickness. As shown more clearly in FIG. 2, the gate insulator layer 16 becomes significantly thinner as it extends over the edges 20 of the polysilicon island 14. In addition, stress-induced inhibition of the thermal oxidation process may result in thin oxide regions along the top edges of the polysilicon. Because the electric field in an insulator, such as the gate insulator 14, is inversely proportional to insulator thickness, the fields in such a thinned region are substantially higher than the average field in the gate insulator 14. As a result, insulation breakdown always occurs prematurely in these thinned regions. In addition, other effects related to the strength of the electric field will be enhanced, including the field aided injection of charge into the insulator, an effect to which a variety of transistor failure and performance degradation problems have been attributed. These problems are generally referred to as "hot-electron effects".
It is desirable to have a thin film field effect transistor whose gate insulator is of essentially uniform thickness and a method of making such a transistor.